NASA's Hubble Space Telescope has provided the most detailed look yet at the comet hurtling toward a July 1994 collision with the giant planet Jupiter. Hubble's high resolution shows that the approximately 20 objects that comprise comet P/Shoemaker-Levy 9 -- giving it the resemblance of a "string of pearls" -- are much smaller than originally estimated from observations with ground-based telescopes. According to Dr. Harold Weaver of the Space Telescope Science Institute (STScI) Baltimore, MD, the Hubble observations show that the cometary nuclei are probably no bigger than three miles across, as opposed to earlier estimates of nine miles.

NASA's Hubble Space Telescope has provided the most detailed look yet at the comet hurtling toward a July 1994 collision with the giant planet Jupiter.

Hubble's high resolution shows that the approximately 20 objects that comprise comet P/Shoemaker-Levy 9- giving it the resemblance of a "string of pearls" - are much smaller than originally estimated from observations with ground-based telescopes. According to Dr. Harold Weaver of the Space Telescope Science Institute (STScI) Baltimore, MD, the Hubble observations show that the cometary nuclei are probably no bigger than three miles across, as opposed to earlier estimates of nine miles.

The new Hubble data show that the impacts will unleash only 1/10th to 1/100th as much energy as thought previously. However, even with these new size estimates, the total energy of the collisions will be equivalent to 100 million megatons of TNT -10,000 times the total destructive power of the world's nuclear arsenal (at the peak of the Cold War). The impacts will be comparable in strength to the collision of a large asteroid or comet with Earth 65 million years ago. This latter cosmic catastrophe is suspected to have caused the extinction of the dinosaurs and hundreds of other species between the Cretaceous and Tertiary ages.

Weaver and a team of co-investigators are announcing their analysis of the Hubble observation at the 25th annual meeting of the Division for Planetary Sciences of the American Astronomical Society held in Boulder, Colorado.

Since the comet's discovery last March, there have been widely varying estimates of how energetic the collisions with Jupiter will be. The force of the collision depends not only on the speed of the impacting bodies, but their size as well.

Measuring the sizes of the nuclei is very difficult because each nucleus is surrounded by a haze of dust, called a coma. "Most of the light being observed is due to scattering by dust in the coma," Weaver says. Relative to ground-based images, the Hubble image provides improved contrast between the nuclei and their comae, thereby allowing a better estimate for the sizes. However, "even the current Hubble image does not allow a clear separation of nucleus and coma, so its size estimates are still probably only upper limits to the true nuclear sizes," according to Weaver.

Fortunately, the definitive answer might be available soon. During the December Hubble Servicing Mission a new camera called WFPC-2, (Wide Field and Planetary Camera 2) with corrective optics to compensate for aberration in Hubble's primary mirror, will be installed on the telescope. "The Hubble repair should provide images with much better contrast than the current images, and if the nuclei are close to the sizes we now think they are, then they should really pop out in the new Hubble images," Weaver says.

Hubble's sharp resolution shows that one bright knot in the comet stream is really four fragments close together. Two of the pieces have an apparent separation of only 700 miles. The Hubble image also shows that most of the visible nuclei have comparable sizes. Weaver says that the close match in size among the chunks suggests they might be the primordial "building blocks" of comets. According to calculations, the parent comet broke apart when it passed close to Jupiter in July 1992. "Jupiter's gravity might have disassembled the comet back into the primordial objects, called planetesimals, that were present when our Sun formed 4.5 billion years ago," Weaver says. "However, since the current Hubble observations cannot detect nuclei much smaller than about 2 km, the size distribution of the planetesimals is still indeterminate. Once Hubble's optics are fixed, we should get a better handle on the range of sizes within the planetesimal population."

Though commonly referred to as a comet, some astronomers think P/Shoemaker-Levy 9 might be an asteroid. In this case, it would have come from the asteroid belt between the orbits of Jupiter and Mars, rather than from a hypothetical comet belt beyond Pluto's orbit. However, no one has ever seen an asteroid break apart so it is difficult to predict how asteroids should behave under these circumstances. Likewise, since there are few detailed studies of comets as far away as Jupiter (1/2 billion miles), it's hard to know how a comet should behave at Jupiter's distance.

At Jupiter's distance the comet's surface is so cold that the sublimation rate of water ice is very small. "On the other hand, the breakup of the comet may have released an unusually large number of icy grains, exposing such a large surface area to the sun that the sublimation might become detectable," Weaver says. "Also, there are probably substances present that are more volatile than water ice."

Weaver's team took spectra near the brightest fragment to search for molecules that might have been released from subliming ice. This would provide strong evidence that P/Shoemaker-Levy 9 is a comet, not an asteroid. Spectroscopic observations made with Hubble Faint Object Spectrograph failed to find hydroxyl molecules that would be a clear indicator of cometary origin.

Another way to address this mystery is by watching the evolution of the surrounding coma. A cometary origin would be likely if the coma is continually replenished by gas streaming off the fragments, since comets are more icy than asteroids. However, if the coma simply spreads out, eventually completely disappearing, the coma might just be dust from a broken-up asteroid. The current Hubble image shows that the coma is apparently not continually being replenished, but more observations are needed to monitor the coma development further.

The Jupiter collision is expected to occur over a six-day period around July 21, 1994. The effect of the impact will depend not only on the size and velocity of the cometary nuclei, but also their composition and structure. Comets are very porous and, thus, might break up high in the atmosphere. For example, on June 30, 1908 a 160-foot (50-meter) wide cometary nucleus or stony meteor is suspected to have disintegrated in Earth's atmosphere at an altitude of five miles (8 km). The resulting explosion leveled hundreds of thousands of acres of forest in Siberia's Tunguska River Valley, Russia.

The Jupiter impacts could potentially produce spectacular phenomena in the giant planet's multicolor cloud tops. The plummeting comet nuclei would turn into gigantic versions of meteors or "shooting stars." Each 100-mile wide, blue-white fireball would blow a hole in Jupiter's atmosphere the size of Texas. Although the impacts are predicted to occur on Jupiter's far side (not observable from Earth), it's likely that the effects on the atmosphere still will be visible as the impact zone rotates into the Earth's view. (Jupiter's rotation rate is 9 hours, 50 minutes.)

The Hubble telescope is expected to be a key player during next year's encounter, although there are no definite observing plans yet. Due to its low-level of scattered light and high angular resolution, the Hubble should be able to observe the comet even when the glare of Jupiter prevents further ground-based observations. After the impact, the Hubble images should show details in Jupiter's atmosphere that are unattainable by any other means.

The comet was discovered last March by Dr. Carolyn Shoemaker of Northern Arizona University, Dr. Eugene Shoemaker of the U.S. Geological Survey, and veteran amateur comet observer David Levy. The HST observing team consists of these three people, Weaver, and 15 others from a variety of institutions.